1. Field of the Invention
The present invention relates to a bidirectional line switched ring (BLSR) network, more particularly relates to a function of generating a path alarm indication signal (AIS) essential for that network.
One of the so-call synchronous digital hierarchy networks (SDH) now being established as a standard in North America is the SONET (synchronous optical network) system. In a ring network of the SONET system, optical fibers are paired to form a ring. One of the two optical fibers is used to perform data transmission in the clockwise direction, while the other is used to perform data transmission in the counterclockwise direction. The standard calls for each optical fiber to include working line time slots and protection line time slots. When a line disconnection or other failure occurs, the working line time slots of one optical fiber are looped back to the protection line time slots of the other optical fiber so as to restore the line and maintain communication in the ring network.
In recent years, there has been a demand in digital optical communication networks for improvement of the rate of line use and the rate of line recovery due to the increased volume of transmission. The BLSR network has been developed in answer to that demand.
In a ring network, for example, data is transferred in units of frames called synchronous transport signals-1 (STS-1). An STS-1 frame is divided roughly into overhead (OH) bytes and data conveyor bytes known as the synchronous payload envelope (SPE).
When some sort of transmission failure occurs in the ring network, an STS-1 frame is transmitted converted to a path-AIS so as to inform the low order group side of the failure. A path-AIS corresponds to the data conveyor bytes in the STS-1 frame rewritten to all "1" data. Accordingly, this path-AIS plays the important role to reduce the impact of the transmission failure to a minimum.
2. Description of the Related Art
As will be explained later in more detail with reference to the drawings, according to the related art, it was necessary to provide each node with maps for the path-AIS generation function. The first problem therefore was that the hardware, in particular the memory, became large in size due to the fact that there were as many as 16 nodes requiring maps and it was necessary to provide each with table information, comprised of 48 channels' (48 time slots') worth of combinations in the case of for example an optical carrier (OC)-48 transmission mode.
Further, the standard calls for the line switching to be completed within 50 ms when a usual loopback switching is to be carried out, and also within 100 ms when an erroneous channel connection occurs and the path-AIS is to be inserted, but if the central processing unit (CPU) has to perform software processing referring to the above-mentioned huge amount of table information, a considerably long time would end up being required until the command for generation of the path-AIS could be issued. That is, there was the second problem of a large time lag until the issuance of the command for generation of the path-AIS.